Evolution of the microstructure of a 15-5PH martensitic stainless steel during precipitation hardening heat treatment

Highlights

• The multi-scale microstructure is characterized along precipitation heat treatment.

• Austenite reverts during precipitation treatment.

• Cu precipitates form with a core-shell structure with a Ni, Mn, Si enriched shell.

• Cr segregates on dislocations during heat treatment.

Abstract

The mechanical properties of precipitation-hardened stainless steels rely on a complex multi-scale microstructure developed during a sequence of quenching after austenitization, followed by a precipitation heat treatment. Important features of the resulting microstructure include the microstructure of martensite, retained and reverted austenite, nanoscale precipitation and the homogeneity of the Cr concentration. In this paper, the microstructure of a Cu-bearing 15-5PH steel is thoroughly characterized along the precipitation heat treatment, using a combination of transmission electron microscopy with phase and orientation mapping, atom probe tomography, in-situ small-angle X-ray scattering and X-ray diffraction. The fraction of austenite is observed to increase during the ageing treatment, together with the precipitation of the Cu precipitates, which present a core-shell structure with a shell enriched in Ni, Mn and Si. After heat treatment, the Cr concentration is found to be slightly inhomogeneous in the matrix, with some segregations at the dislocations.

Introduction

Precipitation hardened stainless steels are used for a wide range of applications requiring the combination of high strength, good toughness and corrosion resistance [1]. In most cases their process route involves the formation of a martensitic microstructure during a quench from the austenite field, followed by an ageing treatment where precipitation occurs. Depending on the alloy, this precipitation involves most classically Cu, such as in the 15-5 Precipitation hardening (PH) or 17-4PH alloys, NiAl such as in the 13-8PH alloy or even Ni₃(Mo,Ti). Tailoring the characteristics of this precipitation in terms of volume fraction and size allows adjusting the balance of the steel in terms of yield strength (or ultimate tensile strength) and fracture toughness [2], [3]. Thus, the Cu precipitation sequence in Fe based alloys [1], [4], [5], [6], [7], [8] and its impact on mechanical properties of PH stainless steels 17-4PH and 15-5PH [3], [9], [10], [11], [12], [13], [14] has been thoroughly investigated during the last decades.

However, the microstructure of a steel such as the 15-5PH at the end of the precipitation ageing treatment is very complex, involving features at different scales, that depend on each step of the thermal history, which can each have determining influences on the materials' properties and on their subsequent evolution during in-service ageing:

• The martensitic structure inherited from the quench may evolve during the ageing treatment, with modifications of the dislocation density and/or lath structure.

• The alloy is not 100% martensitic and austenite may be present after the quench (retained austenite) as well as develop during the ageing treatment (reverted austenite) [15], [16]. In the latter case, there can be complex solute partitioning and interaction with the other microstructural features.

• Precipitation of Cu can interact with the other solutes present in the material such as Ni, Mn, Si: these elements may be partitioned during the formation of the precipitates and therefore influence their formation.

• The presence of other precipitates such as carbides is also of interest.

• The Fe-Cr solid solution may not be entirely homogeneous at the end of the ageing treatment.

The present contribution aims at determining the role of the ageing treatment on the development of these different microstructural features through a multi-scale characterization procedure carried out both in the as-quenched state and during, or after the ageing treatment. The grain structure will be evaluated at fine scale using automated crystal orientation mapping (ACOM) in the transmission electron microscope (TEM). In addition, the fraction of austenite will be evaluated by X-ray diffraction (XRD). The precipitate microstructure will be evaluated in-situ during the ageing treatment using small-angle X-ray scattering (SAXS), supported by TEM. Atom probe tomography (APT) will be used to evaluate the distribution of chemical species in the precipitates and in their vicinity, as well as to evaluate the distribution of Cr atoms in the matrix, and in a particular case to determine the partitioning between austenite and martensite.